Electronic devices such as computers, personal digital assistants (PDA)s, radiotelephones, telecommunications equipment, servers and the like continue to evolve as manufacturers of such devices continue to improve the speed, size, functionality, battery life and longevity of these devices. In such devices it often important to know how much current is being supplied by, or consumed by circuits, sub-circuits and components within the device. These components are typically mounted on a circuit board that interconnects the components utilizing copper traces that are “sandwiched” between layers of insulating circuit board material. During the manufacture of circuit boards, the copper traces can be selectively etched from sheets of copper that are affixed to the insulating layers thereby forming traces.
There are many circumstances where it is desirable to be able to accurately detect relatively high currents in a circuit. The ability to accurately monitor large currents allows for better control of circuit operation. For example, providing precision currents in a battery charging process can greatly increase battery life for battery powered devices. Determining if circuits or components are drawing excessive power can lead to measures that control these circuits such that they draw less power. Such control can greatly increase the reliability of electronic devices. Detecting current can also pinpoint systems and components that are malfunctioning or overheating and such systems can be shut down to avoid a catastrophic failure.
Traditional current sensing systems that sense relatively high currents introduce many design problems and have limited accuracy. For example, the power resistor typically utilized in current sensing systems will typically produce a significant voltage drop in the power line, consume a significant amount of power and the resistance of such a device can change with temperature at an unpredictable rate making an accurate current reading nearly impossible. Thus, a designer is almost always faced with the design challenge of introducing an intrusive power loss and compensating for an intrusive voltage drop. The design challenges in current sensing technology have intensified with the new, low voltage standards. Some electrical components now require high currents at very low supply voltages. Current standards include power requirement of three and a third volts (3.3V) and one volt (1V).
Most traditional current sensing methods insert a precision resistor in series with a power line and the voltage sensing circuits measures the voltage drop across the sense resistor to determine the current. For larger currents, a current sense circuit requires a precision, low resistance, high power resistor. Precision resistors are relatively expensive and today the lowest commercially available resistance value in a precision resistor is a 1 milliohm having a 1% deviation. Often, there is a very little margin to accommodate the voltage drop that is inherent in traditional current sensing resistors in traditional current sensing configurations. Some electronic components such as a microprocessor can require as much as 100 amps at one volt, with extreme voltage sensitivity. For example, if a power trace supplies a regulated one (1) volt supply voltage at one hundred (100) Amps to a microprocessor, the voltage drop across the current sense resistor will reach one hundred (100) millivolts which is 10% of the supply voltage. Thus, in such a design, 10% of the power is wasted on determining how much current the microprocessor is drawing and due to the intrusive voltage drop, the power supply or a voltage regulator module needs to regulate out this voltage drop to ensure proper operation of the microprocessor.